The invention relates to a polymer, producing method thereof, and photorefractive composition. More particularly, the invention relates to polymers and copolymers that include functional groups that provide photorefractive capabilities, and to methods of making such polymers.
Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by laser beam irradiation. The change of refractive index is achieved by a series of steps, including: (1) charge generation by laser irradiation, (2) charge transport, resulting in separation of positive and negative charges, and (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization, and (5) refractive index change induced by the non-uniform electric field.
Therefore, good photorefractive properties can be seen only for materials that combine good charge generation, good charge transport, or photoconductivity, and good electro-optical activity.
Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition.
Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO3. In these materials, the mechanism of the refractive index modulation by the internal space-charge field is based on a linear electro-optical effect.
In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264, to Ducharme et al. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable depending on the application include sufficiently long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.
In recent years, efforts have been made to optimize the properties of organic, and particularly polymeric, photorefractive materials. As mentioned above, good photorefractive properties depend upon good charge generation, good charge transport, also known as photoconductivity, and good electro-optical activity. Various studies that examine the selection and combination of the components that give rise to each of these features have been done. The photoconductive capability is frequently provided by incorporating materials containing carbazole groups. Phenyl amine groups can also be used for the charge transport part of the material.
Non-linear optical ability is generally provided by including chromophore compounds, such as an azo-type dye, which can absorb photon radiation. The chromophore may also provide adequate charge generation. Alternatively, a material known as a sensitizer may be added to provide or boost the mobile charge required for photorefractivity to occur. Many materials, including wide range of dyes and pigments, can serve as sensitizers.
The photorefractive composition may be made simply by mixing the molecular components that provide the individual properties required into a host polymer matrix. However, most compositions prepared in this way are not stable over time, because phase separation tends to occur as the components crystallize or phase separate.
Efforts have been made, therefore, to make polymers that include one or more of the active components in the polymer structure.
A major improvement was to replace the inert polymer matrix by the photoconductive polymer poly(N-vinylcarbazole) (PVK). This allowed the concentration of the charge-transport agent to be increased, while completely excluding crystallization of the carbazole groups. This breakthrough, achieved at the University of Arizona, is reported by N. Peyghambarian et al. (Nature, 1994, 371, 497).
In this case, a photorefractive composition was made by adding an azo dye (DMNPAA; 2,5-dimethyl-4-(p-nitrophenylazo) anisole) as chromophore, and trinitrofluorenone (TNF) as sensitizer. The resulting compositions showed almost 100% diffraction efficiency at laser intensity of 1 W/cm2 and 90 V/xcexcm biased voltage. The response time was slow, however, at over 100 msec.
To achieve good photorefractivity, materials are typically doped with large concentrations of chromophore, such as 25 wt % or more. Thus, crystallization and phase separation of the strongly dipolar chromophore remain a major problem.
To completely eliminate the instability caused by phase separation, it has been recognized that it would be desirable to prepare fully functionalised photorefractive polymers, that is, polymers in which both the photoconductivity and the non-linear optical capability reside within the polymer itself.
Building on the original University of Arizona work, efforts have been made to develop fully functional photorefractive polymers, as well as to speed up the response time. For example, PVK polymers in which some of the carbazole groups are tricyanovinylated have been made (N. Peyghambarian et al., Applied Phys. Lett., 1992, 60, 1803). However, the photoconductivity of this polymer was reported as only 0.98 pS/cm and the diffraction efficiency was less than 1%, too low to show good photorefractivity. Also, the polydispersity of the polymer was high, at 3.3. Subsequently, the same group has reported PVK-based materials with an amazing response time of 4 msec, and a very high photoconductivity of 2,800 pS/cm (N. Peyghambarian et al., J. Mater. Chem., 1999, 9, 2251).
A number of efforts at materials improvement have used methacrylate-based polymers and copolymers that include photoconductive and chromophore side groups. A paper by T. Kawakami and N. Sonoda, (Applied Phys. Lett., 1993, 62, 2167.) discloses acrylate-based polymers containing dicyanovinylideneyl phenylamines as charge transport groups. The diffraction efficiency was reported as around 0.01%.
Japanese Patent Application laid-open JP-A 1995-318992, to Hitachi Ltd. discloses acrylate-based polymers and copolymers made by conventional polymerization techniques and containing charge transport and non-linear-optical groups, but gives no photorefractive performance data.
A report by H. Sato et al., (Technical report of IEICE., 1995, OME-95-53, OPE95-94, 43) describes the preparation of several copolymers having both charge transport components and non-linear optical components in the side groups of the copolymer. However, the charge transport speeds seem to be too slow for good photorefractive materials. The polymers are reported to have polydispersity in the range about 2.3-2.9.
Japanese Patent Application Laid-open JP-A 1998-333195, to Showa Denko, discloses acrylate-based polymers incorporating triphenylamine groups as charge transport agents. Fast response times (50 msec. at 70 V/xcexcm biased voltage) and good polydispersity (1.58) are reported, although there is no description or data regarding diffraction efficiency.
A paper by Van Steenwickel et al. (Macromolecules, 2000, 33, 4074) describes acrylate-based polymers that include carbazole-based side chains and several stilbene-type side chains. The paper cites a high diffraction efficiency of 60% at 58 V/xcexcm, but a slow response time of the sub-second order. Poly dispersity of between 2.5 and 3.8 is reported.
A paper by Y. Chen et al. (Modern Optics, 1999, 46, 1003) discusses a methacrylate polymer that has both carbazole-type side chains to provide charge transport capability and nitrophenyl azo-type side chains to provide non-linear optical capability. The materials again show slow response times of over 20 sec.
None of the materials described above achieves the optimum combination of a high diffraction efficiency with a fast response time, long-term stability and easy processability. Thus, there remains a need for photorefractive compositions that combine these attributes.
In recent years, a new type of polymerization, termed living radical polymerization, has been developed for polymerization of functional monomers, including methacrylate, and styrene derivatives. Living radical polymerization differs from conventional radical polymerization in that the polymer growth terminals can be temporarily protected by protection bonding. This enables polymerization to be well controlled, including being stopped and started at will.
This process can be used to prepare homopolymers and copolymers, including block copolymers. Details of the living radical polymerization method are described in the literature. They may be found, for example, in the following papers:
1. T. Patten et al., xe2x80x9cRadical polymerization yielding polymers with Mw/Mn xcx9c1.05 by homogeneous atom transfer radical polymerizationxe2x80x9d, Polymer Preprints, 1996, 37, 575.
2. Matyjasewski et al., xe2x80x9cControlled/living radical polymerization. Halogen atom transfer radical polymerization promoted by a Cu(I)/Cu(II) redox processxe2x80x9d, Macromolecules, 1995, 28, 7901.
3. M. Sawamoto et al., xe2x80x9cRuthenium-mediated living radical polymerization of methyl methacrylatexe2x80x9d, Macromolecules, 1996, 29, 1070.
Living radical polymerization is also described in U.S. Pat. No. 5,807,937 to Carnegie-Mellon University, which is incorporated herein by reference in its entirety.
As an example of block copolymers prepared by living radical polymerization, novel styrene and butyl acrylate block copolymers for pressure sensitive adhesives have been reported (JP-A 2001-115124, M. Yamamoto et al.). Such block copolymers could not be prepared by conventional polymerization methods.
The only example known to the present inventors of photorefractive polymer preparation by living radical polymerization is in a paper by E. Hattemer et al. (Polymer Preprints, Am. Chem. Soc., Div. Polym. Chem., 2000, 41, 785). This reference discloses the polymerization of a triphenylamine-type styrene monomer, using a phenylethoxy-tetramethylpiperidine (TEMPO-type) initiator. The resulting polymers have low polydispersities of 1.2-1.4. No photorefractive or electro-optical performance data are reported in the citation.
The object of the present invention is to provide a photorefractive composition which exhibits high photoconductivity, a polymer which is desirably used for the photorefractive composition, and producing method thereof.
A first aspect of the present invention is a polymer which is represented by a formula selected from the group consisting of formulae (I), (II), (III) and (IV): 
wherein R0 represents a hydrogen atom or alkyl group with up to 10 carbons; R is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; A represents a repeating structure comprising at least one of the below repeating unit 1 and repeating unit 2; 
wherein R0 represents each independently a hydrogen atom or alkyl group with up to 10 carbons; p is an integer of 2 to 6; A and Axe2x80x2 represents each independently a repeating structure comprising at least one of the below repeating unit 1 and repeating unit 2; 
wherein R0 represents a hydrogen atom or alkyl group with up to 10 carbons; A represents a repeating structure comprising at least one of the below repeating unit 1 and repeating unit 2; 
wherein A represents a repeating structure comprising at least one of the below repeating unit 1 and repeating unit 2; 
wherein Z is represented by a structure selected from the group consisting of structures (i), (ii) and (iii); and Zxe2x80x2 is represented by formula (0); 
wherein Q represents an alkylene group, with or without a hetero atom; such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; R1 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and preferably R1 is an alkyl group which is selected from methyl, ethyl, propyl, butyl, pentyl and hexyl; G is a group having a bridge of xcfx80-conjugated bond; and Eacpt is an electron acceptor group;
wherein the structures (i), (ii) and (iii) are: 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; and wherein Ra1, Ra2, Ra3, Ra4, Ra5, Ra6, Ra7, and Ra8 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; and wherein Rb1-Rb27 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; and 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6, and wherein Rc1-Rc14 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.
A second aspect of the present invention is a method for producing a polymer comprising polymerizing a monomer by a living radical polymerization technique, wherein the monomer comprises a structure selected from the group consisting of the above structures (i), (ii) and (iii).
A third aspect of the present invention is a method for producing a polymer comprising polymerizing a monomer by a living radical polymerization technique, wherein the monomer comprises a structure represented by the above formula (0).
A fourth aspect of the present invention is a method for producing a polymer comprising copolymerizing at least a first monomer and a second monomer by a living radical polymerization technique, wherein the first monomer comprises a structure selected from the group consisting of the above structures (i), (ii) and (iii), and wherein the second monomer comprises a structure represented by the above formula (0).
A fifth aspect of the present invention is a composition comprising a sensitizer and a polymer according to the first aspect of the present invention, wherein the composition exhibits photorefractive ability.
A sixth aspect of the present invention is a composition comprising a polymer prepared by living radical polymerization, wherein: (a) the living radical polymerization is carried out using a monomer, a polymerization initiator, transition metal catalyst and a ligand capable of reversibly forming a complex with the transition metal catalyst, (b) the polymer comprises at least one of a first repeat unit including a moiety having charge transport ability and a second repeat unit including a moiety having non-linear-optical ability, and (c) the composition exhibits photorefractive ability. One or both of the photoconductive (charge transport) and non-linear optical components are incorporated into the chemical structure of the polymer itself, typically as side groups.
The composition differs from photorefractive compositions previously known in the art in several points. In a first point, it is prepared by living radical polymerization, preferably by using a transition metal catalyst.
In a second point, the composition provides high photoconductivity compared with prior art photoconductive materials, and/or one or more other advantageous properties, such as high diffraction efficiency, fast response time, low polydispersity, and comparatively low glass transition temperature. Furthermore these properties can typically be provided in conjunction with one or more other desirable attributes, such as stability, that is, resistance to phase separation, low viscosity, and excellent handling and processing capability.
In a third point, the composition comprises a random copolymer or a block copolymer incorporating blocks containing the photoconductive side group and blocks containing the non-linear-optical (chromophore) side group.
In a fourth point, the composition is characterized by a low polydispersity compared with typical polymers, and combines low polydispersity with one or more good photorefractive properties, such as high diffraction efficiency, high photoconductivity and fast response time. Furthermore, as above, these properties can typically be provided in conjunction with one or more other desirable attributes, such as stability, low viscosity, and excellent handling and processing capability.
With respect to the first point of the invention, it was discovered by the inventors that living radical polymerization techniques could be adapted to provide polymers with improved properties for use in photorefractive compositions. Living radical polymerization technique by the inventors makes available to the art a number of innovative features, including use of acrylate-based monomers incorporating charge transport groups and/or non-linear-optical (chromophore) groups, use of transition metal catalyst systems for preparation of photorefractive materials, and use of a monomer incorporating a chromophore precursor group.
With respect to the second point of the invention, it was found by the inventors that photorefractive compositions of the present invention exhibit high photoconductivity. High photoconductivity is supposed to contribute to high response time. As explained in more detail below, photoconductivity is the increase in conductivity of a material when exposed to light. Conventional organic photorefractive materials typically exhibit photoconductivities in the range 0.01-1,000 pS/cm, with most having photoconductivity no higher than about 10 pS/cm. Materials of the present invention are typically able to exhibit higher photoconductivity of at least about 100 pS/cm, preferably about 200 pS/cm or more, and more preferably about 500 pS/cm or more.
To inventors"" knowledge, the highest photoconductivity that has ever been reported for a photorefractive material is 2,800 pS/cm, in a PVK-type material. However materials with photoconductivity above 3,000 pS/cm have been produced by the inventors, and specifically as high as 3,500 pS/cm.
Also, unlike polycarbazole, which becomes viscous and sticky during processing, materials of the present invention retain their low viscosity and have excellent handling properties for standard device fabrication techniques, such as injection molding, extrusion, and various film-forming processes.
Furthermore, materials of the present invention combine exceptional photoconductivity with very fast response times, such as 50 ms or less.
With respect to the third point of the invention, the first photorefractive block copolymers in the art was developed by the inventors. The block copolymers of the invention can manifest essentially any combination of blocks containing units with photoconductive (charge transport) ability and blocks containing units with non-linear optical ability. For example, if A represents a polymer block that incorporates charge transport ability groups and B represents a polymer block that incorporates non-linear-optical ability groups, then the copolymers of the invention include any combinations of A and B units. As representative, but non-limiting examples, polymers of the forms Axe2x80x94B, Bxe2x80x94A, Axe2x80x94Bxe2x80x94A, Bxe2x80x94Axe2x80x94B, and so on, are included.
Also included are copolymers that include more diverse monomer units, such as A1xe2x80x94A2xe2x80x94B, A1xe2x80x94B1xe2x80x94A2xe2x80x94B2 and so on, where A1 and A2 represent different types of A polymer and B1 and B2 represent different types of B polymer.
The block copolymers of the invention can be readily made by the adapted living radical polymerization techniques discovered by the inventors.
Random-type copolymers comprising at least two functional groups, a charge transport functional group and a non-linear optical functional group, are also provided, and can be readily made by the processes of the invention.
Both the random copolymers and the block copolymers disclosed herein provide the advantage of long-term stability, due to lower likelihood of phase separation or crystallization, compared with polymer materials in which the functionality is provided by adding functional materials in the form of dopants.
With respect to the fourth point of the invention, the polymers of the invention exhibit low polydispersity. As discussed in more detail below, polydispersity is a measure of the spread of molecular weights of the molecules in a polymer. In general, the lower the polydispersity, the more uniform are the bulk physical properties of the polymer, such as thermal and mechanical characteristics. This is an important feature for the performance of photorefractive compositions. Typically, polymers prepared by conventional techniques exhibit a polydispersity of greater than about 2.5. According to the present invention, materials that typically exhibit a polydispersity of about 2.5 or below, and preferably about 2.0 or below, are provided.
Furthermore, for polymers having the same chemical structure and average molecular weight, a polymer composition of low polydispersity exhibits a lower glass transition temperature than the corresponding composition of higher polydispersity. For example, typical glass transition temperature for the types of methacrylate polymers preferred for polymer compositions of the invention is about 90-120xc2x0 C. when made by conventional polymerization techniques. When made by the living radical polymerization technique taught herein, the glass transition temperature of the polymer is typically between about 80xc2x0 C. and 100xc2x0 C. This is important, because a lower glass transition temperature tends to lead to better photorefractive performances, such as high photoconductivity, fast response time and high diffraction efficiency.
The comparatively lower Tg of polymers of the invention reduces the dependence on large amounts of plasticizer in the finished composition. This improves the handling properties of the composition and the long-term stability.
These materials of comparatively low polydispersity may be readily prepared by living radical polymerization technique of the invention.
The photorefractive composition comprises a polymer matrix, and includes a component that provides photoconductive or charge transport ability and a component that provides non-linear optical ability. Optionally, the composition may also include other components as desired, such as sensitizer and plasticizer components.
One or both of the photoconductive and non-linear optical components are incorporated as functional groups into the polymer structure, typically as side groups.
The group that provides the charge transport or photoconductive functionality may be any group known in the art to provide such capability. If this group is to be attached to the polymer matrix as a side chain, then the group should be capable of incorporation into a monomer that can be polymerized to form the polymer matrix of the composition.
Preferred photoconductive groups are phenyl amine derivatives, particularly carbazoles and di- and tri-phenyl diamine.
Most preferably the moiety that provides the photoconductive functionality is chosen from the group of phenyl amine derivatives consisting of the following side chain structures (i) to (iii): 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; and wherein Ra1, Ra2, Ra3, Ra4, Ra5, Ra6, Ra7, and Ra8 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; and wherein Rb1-Rb27 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; and 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6, and wherein Rc1-Rc14 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.
Likewise, the chromophore or group that provides the non-linear optical functionality may be any group known in the art to provide such capability. If this group is to be attached to the polymer matrix as a side chain, then the group, or a precursor of the group, should be capable of incorporation into a monomer that can be polymerized to form the polymer matrix of the composition.
The chromophore or group that provides the non-linear optical functionality used in the present invention is represented by formula (0): 
wherein Q represents an alkylene group, with or without a hetero atom; such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; R1 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and preferably R1 is an alkyl group which is selected from methyl, ethyl, propyl, butyl, pentyl and hexyl; G is a group having a bridge of xcfx80-conjugated bond; and Eacpt is an electron acceptor group.
In the above definition, by the term xe2x80x9ca bridge of xcfx80-conjugated bondxe2x80x9d, it is meant a molecular fragment that connects two or more chemical groups by xcfx80-conjugated bond. A xcfx80-conjugated bond contains covalent bonds between atoms that have "sgr" bonds and xcfx80 bonds formed between two atoms by overlap of their atomic orbitals (s+p hybrid atomic orbitals for "sgr" bonds; p atomic orbitals for xcfx80 bonds).
By the term xe2x80x9celectron acceptorxe2x80x9d, it is meant a group of atoms with a high electron affinity that can be bonded to a xcfx80-conjugated bridge. Exemplary acceptors, in order of increasing strength, are:
C(O)NR2 less than C(O)NHR less than C(O)NH2 less than C(O)OR less than C(O)OH less than C(O)R less than C(O)H less than CN less than S(O)2R less than NO2 
As typical exemplary electron acceptor groups, functional groups which is described in prior of art U.S. Pat. No. 6,267,913 and shown in the following structure figure can be used. U.S. Pat. No. 6,267,913 is hereby incorporated by reference for the purpose of describing donors and acceptors useful in this invention. The symbol xe2x80x9c‡xe2x80x9d in a chemical structure herein specifies an atom of attachment to another chemical group and indicates that the structure is missing a hydrogen that would normally be implied by the structure in the absence of the xe2x80x9c‡xe2x80x9d. 
wherein R is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons.
Preferred chromophore groups are aniline-type groups or dehydronaphtyl amine groups.
Most preferably the moiety that provides the non-linear optical functionality is such a case that G in formula (0) is represented by a structure selected from the group consisting of the structures (iv), (v) and (vi); 
wherein, in both structures (iv) and (v), Rd1-Rd4 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons, and preferably Rd1-Rd4 are all hydrogen; R2 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons; 
wherein R7 represents a linear or branched alkyl group with up to 10 carbons; and
wherein Eacpt in formula (0) is an electron acceptor group and represented by a structure selected from the group consisting of the structures; 
wherein R9, R10, R11 and R12 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons.
A preferred polymer used for the photorefractive composition is the following formulae (Ia), (IIa), (IIIa), (IVa), (Ib), (IIb), (IIIb), (IVb), (Ic), (IIc), (IIIc) and (IVc): 
wherein R0, R and Z are the same meaning as in formula (I); and n is an integer of 10 to 10,000; 
wherein R0, R and Z are the same meaning as in formula (II); and m and n are an integer of 5 to 10,000, respectively; 
wherein R0 and Z are the same meaning as in formula (III); and n is an integer of 10 to 10,000; 
wherein Z is the same meaning as in formula (IV); and n is an integer of 10 to 10,000; 
wherein R0, R and Zxe2x80x2 are the same meaning as in formula (I); and n is an integer of 10 to 10,000; 
wherein R0, R and Zxe2x80x2 are the same meaning as in formula (II); and m and n are an integer of 5 to 10,000, respectively; 
wherein R0 and Zxe2x80x2 are the same meaning as in formula (III); and n is an integer of 10 to 10,000; 
wherein Zxe2x80x2 is the same meaning as in formula (IV); and n is an integer of 10 to 10,000; 
wherein R0, R, Z and Zxe2x80x2 are the same meaning as in formula (I); x is an integer of 5 to 10,000; and y is an integer of 5 to 10,000; 
wherein R0, R, Z and Zxe2x80x2 are the same meaning as in formula (II); x is an integer of 5 to 10,000; y is an integer of 5 to 10,000; r is an integer of 5 to 10,000; and s is an integer of 5 to 10,000; 
wherein R0, Z and Zxe2x80x2 are the same meaning as in formula (III); and x is an integer of 5 to 10,000; and y is an integer of 5 to 10,000; 
wherein Z and Zxe2x80x2 are the same meaning as in formula (IV); and x is an integer of 5 to 10,000; and y is an integer of 5 to 10,000.
The polymer matrix is preferably synthesized from a monomer incorporating at least one of the above photoconductive groups or one of the above chromophore groups. The inventors have recognized that a number of physical and chemical properties are desirable in the polymer matrix. It is preferred if the polymer itself incorporates both a charge transport group and a chromophore group, so the ability of the monomer units to form copolymers is preferred. Physical properties of the formed polymer that are of importance are the molecular weight or more specifically the molecular weight distribution, as reflected in the polydispersity, and the glass transition temperature, Tg. Also, it is valuable and desirable, although not essential, that the polymer should be capable of being formed into films, coatings and shaped bodies of various kinds by standard polymer processing techniques, such as solvent coating, injection molding and extrusion.
In the present invention, the polymer generally has a weight average molecular weight, Mw, of from about 3,000 to 500,000, preferably from about 5,000 to 100,000. The term xe2x80x9cweight average molecular weightxe2x80x9d as used herein means the value determined by the GPC (gel permeation chromatography) method in polystyrene standards, as is well known in the art.
More significantly, the polymer preferably has a narrow polydispersity compared with typical polymers. By a narrow polydispersity, it means a polydispersity less than about 2.5, and more preferably less than about 2.0. For the present purposes, polydispersity is given by the ratio Mw/Mn (in which Mw is as defined above, and Mn is number average molecular weight, also determined by GPC in a polystyrene standard).
Polydispersity is important because of its correlation to polymer properties, such as viscosity, glass transition temperature, and other thermal and mechanical properties. Even when a polymer has the same chemical structure and components, a matrix of low polydispersity will tend to have a lower viscosity, and better thermal and mechanical handling properties, than a matrix of the same chemical structure but higher polydispersity. The low viscosity can also give rise to improved photorefractive properties.
For good photorefractive properties, the photorefractive composition should be substantially amorphous and non-crystalline under the conditions of use. Therefore, it is preferred that the finished photorefractive composition has a relatively low glass transition temperature, Tg, such as below about 50xc2x0 C., more preferably below about 40xc2x0 C. Preferred temperature ranges for the Tg are 10-50xc2x0 C., most preferably 20-40xc2x0 C. If the pure polymer itself has a glass transition temperature higher than these preferred values, which will generally be the case, components may be added to lower the Tg, as discussed in more detail below.
Nevertheless, it is preferred that the polymer itself has a relatively low glass transition temperature, by which the inventors mean a Tg no higher than about 125xc2x0 C., more preferably no higher than about 120xc2x0 C., and most preferably no higher than about 110xc2x0 C. or 100xc2x0 C.
A relatively low glass transition temperature is preferred because the greater mobility of polymer chains that polymers exhibit close to or above their glass transition temperature gives higher orientation during voltage application, and leads to better performance, such as high photoconductivity, fast response time and high diffraction efficiency, of the photorefractive device.
As mentioned in the Summary section above, a polymer composition of low polydispersity exhibits a lower glass transition temperature than the corresponding composition of higher polydispersity. For example, typical glass transition temperature for the types of (meth)acrylate polymers preferred for polymer compositions of the invention is about 90-120xc2x0 C. when made by conventional polymerization techniques. When made by the low-polydispersity methods taught herein, the glass transition temperature is typically between about 80xc2x0 C. and 100xc2x0 C.
In principle, essentially any polymer backbone, including, but not limited to, polyurethane, epoxy polymers, polystyrene, polyether, polyester, polyamide, polyimide, polysiloxane, and polyacrylate could be used, with the appropriate side chains attached, to make the polymer matrices of the invention.
Preferred types of backbone units are those based on (meth)acrylates or styrene. Particularly preferred are methacrylate-based monomers, and most preferred are acrylate monomers. The first polymeric materials to include photoconductive functionality in the polymer itself were the polyvinyl carbazole materials developed at the University of Arizona. However, these polyvinyl carbazole polymers tend to become viscous and sticky when subjected to the heat-processing methods typically used to form the polymer into films or other shapes for use in photorefractive devices.
In contrast, preferred materials of the present invention, and particularly the (meth)acrylate-based, and more specifically methacrylate-based, polymers, have much better thermal and mechanical properties. That is, they provide better workability during processing by injection-molding or extrusion, for example. This is particularly true when the polymers are prepared by living radical polymerization, as described below, since this method yields a polymer product of lower viscosity than would be the case for the same polymer prepared by other methods.
Particular examples of monomers including a phenyl amine derivative group as the charge transport component are carbazolylpropyl (meth)acrylate monomer; 4-(N,N-diphenylamino)-phenylpropyl(meth)acrylate; N-[(meth)acroyloxypropylphenyl]-N,Nxe2x80x2,Nxe2x80x2-triphenyl-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; N-[(meth)acroyloxypropylphenyl]-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(3-methylphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; and N-[(meth)acroyloxypropylphenyl]-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-buthoxyphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine. Such monomers can be used singly or in mixtures of two or more monomers.
Particular examples of monomers including a chromophore group as the non-linear optical component are N-ethyl, N-4-dicyanomethylidenyl acrylate and N-ethyl, N-4-dicyanomethylidenyl-3,4,5,6,10-pentahydronaphtylpentyl acrylate.
In light of the desired features mentioned above, the inventors have recognized that the recently developed polymerization technique known as living radical polymerization has the potential for preparing polymers with unusually good photorefractive properties. In particular, living radical polymerization has the potential to form polymers with unusually low polydispersity, such as less than 2.5, preferably less than 2.0. Living radical polymerization can also be used to form random copolymers and block copolymers, as discussed in more detail below.
Diverse polymerization techniques are known in the art. One such technique is radical polymerization, which is typically carried out by using an azo-type initiator, such as AEBN (azoisobutyl nitrile).
In conventional radical polymerization, the polymer growth terminal is always in the active radical state, so it is easy for unwanted side reactions to occur, such as bimolecular coupling or disproportionation, generally making it difficult to achieve precise control of polymerization. As a result, this technique is not attractive for preparing photorefractive polymer materials.
On the other hand, as stated above, living radical polymerization is a new technique that offers the opportunity to prepare polymers with properties tailored to achieve improved photorefractive capability. Living radical polymerization differs from conventional radical polymerization in that the polymer growth terminals are temporarily protected by protection bonding. Through reversibly and radically severing this bond, it is possible to control and facilitate the growth of polymer molecules. For example, in a polymerization reaction, an initial supply of monomer can be completely consumed and growth can be temporarily suspended. However, by adding another monomer of the same or different structure, it is possible to restart polymerization. Therefore, the position of functional groups within the polymer can be controlled.
Although various polymerization techniques are known to the art and may be used in the invention, it is preferred, therefore, to prepare the polymer matrix materials of the invention by living radical polymerization, and the inventors have developed customized procedures for so doing.
Details of the living radical polymerization method are described in the literature. They may be found, for example, in the following papers:
T. Patten et al., xe2x80x9cRadical polymerization yielding polymers with Mw/Mn xcx9c1.05 by homogeneous atom transfer radical polymerizationxe2x80x9d, Polymer Preprints, 1996, 37, 575.
K. Matyjasewski et al., xe2x80x9cControlled/living radical polymerization. Halogen atom transfer radical polymerization promoted by a Cu(I)/Cu(II) redox processxe2x80x9d, Macromolecules, 1995, 28, 7901.
M. Sawamoto et al., xe2x80x9cRuthenium-mediated living radical polymerization of methyl methacrylatexe2x80x9d, Macromolecules, 1996, 29, 1070.
Living radical polymerization is also described at length in U.S. Pat. No. 5,807,937 to Carnegie-Mellon University, which is incorporated herein by reference in its entirety.
Briefly, living radical polymerization technique of the invention involves the use of a polymerization initiator, transition metal catalyst and a ligand (an activating agent) capable of reversibly forming a complex with the transition metal catalyst.
The polymerization initiator is typically a halogen-containing organic compounds. After polymerization, this initiator or components of the initiator are attached to the polymer at both polymer terminals. The polymerization initiator preferably used is an ester-based or styrene-based derivative containing a halogen in the xcex1-position.
The polymerization initiator is preferably shown by the following formula (Ixe2x80x3), (IIxe2x80x3) or (IIIxe2x80x3). 
wherein R0 represents a hydrogen atom or alkyl group with up to 10 carbons; and R is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein R0 represents a hydrogen atom or alkyl group with up to 10 carbons. 
Particularly preferred are 2-bromo(or chloro) methylpropionic acid, or bromo-(or chloro)-1-phenyl derivatives. Specific examples of these derivatives include ethyl 2-bromo(or chloro)-2-methylpropionate, ethyl 2-bromo(or chloro)propionate, 2-hydroxyethyl 2-bromo(or chloro)-2-methylpropionate, 2-hydroxyethyl 2-bromo(or chloro)propionate, and 1-phenyl ethyl bromide(chloride).
Instead of a mono bromo(chloro) type initiator, a di-bromo(chloro) type initiator, such as dibromo(chloro) ester derivative, can be used. Such initiators are represented by the formula (IVxe2x80x3): 
wherein R0 represents independently a hydrogen atom or alkyl group with up to 10 carbons; and p is 2 to 6.
Of these initiators, most preferred is ethylene bis(2-bromo (chloro)-2-methylpropionate). By using this initiator, the inventors have discovered that block copolymers, and particularly Axe2x80x94Bxe2x80x94A type or Bxe2x80x94Axe2x80x94B type block copolymers, can be produced very efficiently.
In the process of the invention, the polymerization initiator is generally used in an amount of from 0.01 to 20 mol %, preferably from 0.1 to 10 mol %, and more preferably from 0.2 to 5 mol %, per mole of the sum of the polymerizable monomers.
Various types of catalysts are known, including perfluoroalkyl iodide type, TEMPO (phenylethoxy-tetramethylpiperidine) type, and transition metal type. The inventors have discovered that high-quality polymers can be made by using transition-metal catalysts, which are safer, simpler, and more amenable to industrial-scale operation than TEMPO-type catalysts. Therefore, in the process of the invention a transition-metal catalyst is preferred.
Non-limiting examples of transition metals that may be used include Cu, Ru, Fe, Rh, V, and Ni. Particularly preferred is Cu. Typically, but not necessarily, the transition metal is used in the form of the metal halide (chloride, bromide, etc.).
The transition metal in the form of a halide or the like is generally used in the amount of from 0.01 to 3 moles, and preferably from 0.1 to 1 mole, per mole of polymerization initiator.
The activating agent (ligand) is an organic ligand of the type known in the art that can be reversibly coordinated with the transition metal as a center to form a complex. The ligand preferably used is a bipyridine derivative, mercaptans derivative, trifluorate derivative, or the like. When complexed with the activating ligand, the transition metal catalyst is rendered soluble in the polymerization solvent. In other words, the activating agent serves as a co-catalyst to activate the catalyst, and start the polymerization.
The ligand is used in an amount of normally from 1 to 5 moles, and preferably from 2 to 3 moles, per mole of transition metal halide.
The use of the polymerization initiator and the activating agent in the above recommended proportions makes it possible to provide good results in terms of the reactivity of the living radical polymerization and the molecular weight and weight distribution of the resulting polymer.
In the present invention, living radical polymerization can be carried out without a solvent or in the presence of a solvent, such as butyl acetate, toluene or xylene.
To initiate the polymerization process, the monomer(s), polymerization initiator, transition metal catalyst, activating agent and solvent are introduced into the reaction vessel. As the process starts, the catalyst and initiator form a radical, which attacks the monomer and starts the polymerization growth.
The living radical polymerization is preferably carried out at a temperature of from about 70xc2x0 C. to 130xc2x0 C., and is allowed to continue for about 1 to 100 hours, depending on the desired final molecular weight and polymerization temperature, and taking into account the polymerization rate and deactivation of catalyst.
To perform the polymerization without using a solvent, the reaction is carried out in a similar manner, above the melting point of the monomer. For example, the melting point of a TPD monomer may be 125xc2x0 C., in which case the polymerization may be carried out at 130xc2x0 C.
By carrying out the living radical polymerization technique based on the teachings and preferences given above, it is possible to prepare homopolymers carrying charge transport or non-linear optical groups, as well as random or block copolymers carrying both charge transport and non-linear optical groups. Further, by following the techniques described herein, it is possible to prepare such materials with exceptionally good properties, such as polydispersity, photoconductivity, response time and diffraction efficiency.
If the polymer is made from monomers that provide only charge transport ability, the photorefractive composition of the invention can be made by dispersing a component that possesses non-linear optical properties through the polymer matrix, as is described in U.S. Pat. No. 5,064,264 to IBM, which is incorporated herein by reference. Suitable materials are known in the art and are well described in the literature, such as in D. S. Chemla and J. Zyss, xe2x80x9cNonlinear Optical Properties of Organic Molecules and Crystalsxe2x80x9d (Academic Press, 1987). Also, as described in U.S. Pat. No. 6,090,332 to Seth R. Marder et. al., fused ring bridge, ring locked chromophores that form thermally stable photorefractive compositions can be used. For typical, non-limiting examples of chromophore additives, the following chemical structure compounds can be used: 
The chosen compound(s) is usually mixed in the matrix charge transport homopolymer in a concentration of about 1-80 wt %, more preferably 5-50 wt %.
On the other hand, if the polymer is made from monomers that provide only non-linear optical ability, the photorefractive composition of the invention can be made by mixing a component that possesses charge transport properties into the polymer matrix, again as is described in U.S. Pat. No. 5,064,264 to IBM. Preferred charge transport compounds are good hole transfer compounds, for example N-alkyl carbazole or triphenylamine derivatives.
As an alternative, or in addition, to adding the charge transport component in the form of a dispersion of entities comprising individual molecules with charge transport capability, a polymer blend can be made of individual polymers with charge transport and non-linear optical abilities. For the charge transport polymer, the polymers already described above, such as containing phenyl-amine derivative side chains, can be used. Since polymers containing only charge transport groups are comparatively easy to prepare by conventional techniques, the charge transport polymer may be made by living radical polymerization or by any other convenient method.
To prepare the non-linear optical polymer itself, monomers that have side-chain groups possessing non-linear-optical ability should be used. Non-limiting examples of monomers that may be used are those containing the following chemical structures: 
wherein Q represents an alkylene group with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is of about 2 to 6; R0 is a hydrogen atom or methyl group, and R is a linear or branched alkyl group with up to 10 carbons; and preferably R is an alkyl group which is selected from methyl, ethyl, and propyl.
The inventors have discovered a new technique for preparing such polymers. The technique involves the use of a precursor monomer containing a precursor functional group for non-linear optical ability. Typically, this precursor is represented by the general formula: 
wherein R0 is a hydrogen atom or methyl group, and V is selected from the group consisting of the following structures 1 to 3: 
wherein, in both structures 1 and 2, Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is of about 2 to 6; and wherein Rd1-Rd4 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons, and preferably Rd1-Rd4 are hydrogen; and wherein R1 represents a linear or branched alkyl group with up to 10 carbons, and preferably R1 is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyls; and 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is of about 2 to 6; and wherein R1 represents a linear or branched alkyl group with up to 10 carbons, and preferably R1 is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyls; and wherein R7 represents a linear or branched alkyl group with up to 10 carbons.
The procedure for performing the living radical polymerization in this case involves the use of the same polymerization initiators, transition metal catalysts, activating agents, and solvents, and the same operating conditions and preferences as have already been described above.
After the precursor polymer has been formed, it can be converted into the corresponding polymer having non-linear optical groups and capabilities by a condensation reaction. Typically, the condensation reagent may be selected from the group consisting of 
wherein R9, R10, R11, and R12 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.
The condensation reaction can be done at room temperature for 1-100 hrs, in the presence of a pyridine derivative catalyst. A solvent, such as butyl acetate, chloroform, dichloromethylene, toluene or xylene can be used. Optionally, the reaction may be carried out without the catalyst at a solvent reflux temperature of 30xc2x0 C. or above for about 1 to 100 hours.
The inventors have discovered that use of a monomer containing a precursor group for non-linear-optical ability, and conversion of that group after polymerization tends to result in a polymer product of lower polydispersity than the case if a monomer containing the non-linear-optical group is used. This is, therefore, preferred technique by the invention.
To prepare copolymers, both the non-linear-optical monomer and the charge transport monomer, each of which can be selected from the types mentioned above, should be used.
There are no restrictions on the ratio of monomer units. However, as a typical representative example, the ratio of [a (meth)acrylic monomer having charge transport ability]/[a (meth)acrylate monomer having non-linear optical ability] is between about 4/1 and 1/4 by weight. More preferably, the ratio is between about 2/1 and 1/2 by weight. If this ratio is less than about 1/4, the charge transport ability is weak, and the response time tends to be too slow to give good photorefractivity. On the other hand, if this ratio is more than about 4/1, the non-linear-optical ability is weak, and the diffraction efficiency tends to be too low to give good photorefractivity.
In the living radical polymerization method of the invention, the monomer addition sequence is important for achieving the desired copolymer structure. For example, to make random copolymers, both the chromophore-containing and the charge-transport-group-containing monomers can be added at the same time.
However, by adding the monomers sequentially, block type copolymers can be prepared. For example, to prepare an Axe2x80x94B type block copolymer, wherein polymer block A has charge transport ability and polymer block B has non-linear-optical ability, firstly the monomer having charge transport ability is polymerized, preferably by using a mono bromo(chloro) type initiator. Subsequently, the second monomer having non-linear-optical ability is added to continue the polymerization. In this way, an Axe2x80x94B type block copolymer can be produced. During this polymerization procedure, the second monomer is added at the time when the first monomer is polymerized more than 50% by weight, normally 70% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
On the other hand, if the monomer having non-linear-optical ability is polymerized first, a Bxe2x80x94A type block copolymer can be produced. Similarly to the above polymerization procedure, the second monomer is added at the time when the first monomer is polymerized more than 50% by weight, normally 70% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
Further, if living radical polymerization is carried out in a manner such that, first, the monomer having charge transport ability is polymerized, then the second monomer having non-linear-optical ability is added to continue polymerization, and thirdly an additional amount of the monomer having charge transport ability is added to continue polymerization, an Axe2x80x94Bxe2x80x94A type block copolymer can be produced. During the successive polymerization procedure, the monomer to be subsequently added is added at the time when the conversion of the monomer which has been previously added exceeds at least 50% by weight, normally 60% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
Moreover, if the above three-stage polymerization is followed by the addition of the another monomer to continue the polymerization of monomers, an Axe2x80x94Bxe2x80x94Axe2x80x94B type block copolymer can be produced. From the above explanation, it will be apparent to those of skill in the art that the new methods that the inventors have developed can be used, by changing the sequence of monomer addition, to produce block copolymers of any desired type, including, but not limited to Bxe2x80x94Axe2x80x94B, Bxe2x80x94Axe2x80x94Bxe2x80x94A, Bxe2x80x94Axe2x80x94Bxe2x80x94Axe2x80x94Bxe2x80x94A, or Axe2x80x94Bxe2x80x94Axe2x80x94Bxe2x80x94A type block copolymers.
If the copolymer constitutes two or more of polymer blocks A, the A-type constituting blocks need not necessarily be prepared from the same monomer. Likewise, if the copolymer constitutes two or more of polymer blocks B, the B-type blocks need not necessarily be prepared from the same monomer. Thus, the individual blocks may be of different forms represented by A1, A2, A3, etc. and B1, B2, B3 etc. In this way, a large diversity of polymers, such as A1xe2x80x94Bxe2x80x94A2, B1xe2x80x94B2xe2x80x94A, or A1xe2x80x94B1xe2x80x94A2xe2x80x94B2 can be produced.
Optionally, other components may be added to the polymer matrix to provide or improve the desired physical properties mentioned earlier in this section. Usually, for good photorefractive capability, it is preferred to add a photosensitizer to serve as a charge generator. A wide choice of such photosensitizers is known in the art. Typical, but non-limiting examples of photosensitizers that may be used are 2,4,7-trinitro-9-fluorenone (TNF) and C60. The amount of photosensitizer required is usually less than 3 wt %.
As mentioned above, it is preferred that the polymer matrix have a relatively low glass-transition temperature, and be workable by conventional processing techniques. Optionally, a plasticizer may be added to the composition to reduce the glass transition temperature and/or facilitate workability. The type of plasticizer suitable for use in the invention is not restricted; many such materials will be familiar to those of skill in the art. Representative typical examples include N-alkylcarbazole and dioctylphthalate. Oligomer-type compounds of the charge transport or non-linear-optical monomers may also be used to control the Tg of the composition.
In general, the smallest amount of plasticizer required to provide a suitable overall Tg for the composition should be used. Compositions with large amounts of plasticizer tend to have lower stability, as the polymer matrix and the plasticizer may phase separate over time. Also, the photorefractive properties of the material are diminished by dilution of the active components by the plasticizer.
As discussed above, the invention provides polymers of comparatively low Tg when compared with similar polymers prepared in accordance with prior art methods. The inventors have recognized that this provides a benefit in terms of lower dependence on plasticizers. By selecting polymers of intrinsically moderate Tg and by using methods that tend to depress the average Tg, it is possible to limit the amount of plasticizer required for the composition to preferably no more than about 30% or 25%, and more preferably lower, such as no more than about 20%.
Yet another method to adjust the Tg or improve film formation ability, for example, is to add another monomer, such as an acrylic or methacrylic acid alkyl ester, as a modifying co-monomer. Examples of modifying co-monomers are CH2xe2x95x90CR0xe2x80x94COOR wherein R0 represents a hydrogen atom or methyl group, and R represents a C2-14 alkyl group, such as butylacrylate, ethyl acrylate, propyl acrylate, 2-ethylhexyl (meth)acrylate and hexyl (meth)acrylate.
The photorefractive materials of the invention provide combinations of desirable properties not previously available to the art.
One particularly advantageous feature is the high photoconductivity. In the context of the invention, by photoconductivity the inventors mean the increase in conductivity of the photorefractive material under laser irradiation. The photoconductivity of a sample of material may be measured by the following method. First, the steady-state conductivity properties are measured in the dark, by applying an electric field across the sample, allowing the system to come to steady state, and measuring the resulting current. Then the measurements are repeated while illuminating the sample with a pulse from a single laser beam. The photoconductivity, "PHgr"photo, can then be calculated using the following relation:
"PHgr"photo=(iphotoxe2x88x92idark)/AbeamEa
where:
Ea is the applied electric field,
Abeam is the illuminated area,
iphoto is electric current with laser irradiation, and
idark is electric current without irradiation
Photoconductivity is important because it is a measure of how efficiently charge transport can take place in the material. If all other parameters are fixed, the higher the photoconductivity, the faster is the device response time.
Typical photoconductivities for organic materials and polymers are in the range from about 0.01 pS/cm to a maximum of no higher than 1,000 pS/cm. In fact, the value for conventional photorefractive polymers is usually less than 10 pS/cm, as described in several papers, for example in M. A. Diaz-Garcia et al. (Chem. Mater., 1999, 11, 1784). However, surprisingly, the polymers of the invention generally have photoconductivity of more than 100 pS/cm, which is at least an order of magnitude better than currently used materials, and preferably exhibit a photoconductivity of more than 200 pS/cm.
A very few materials have been reported with photoconductivity higher than 10 pS/cm, and even fewer with photoconductivity higher than 100 pS/cm. To the inventors"" knowledge, the highest photoconductivity ever reported is 2,800 pS/cm (N. Peyghambarian et al., J. Mater. Chem., 1999, 9, 2251), in a PVK-type material.
Yet, for materials of the invention the inventors have measured photoconductivity of 3,000 pS/cm or above, specifically 3,500 pS/cm.
Furthermore, the inventors are aware of no photorefractive composition that provide a photoconductivity of at least 10 pS/cm and, at the same time, offers any one of the other advantageous properties provided herein, such as a polydispersity of no more than about 2.5 or 2.0, a response time of no more than about 50 ms, a diffraction efficiency of at least about 5%, the good mechanical properties and easy processability of an acrylate-based polymer, the presence of both charge transport and non-linear optical groups as side chains in the polymer, especially in the form of block copolymers, or the efficiency and flexibility of preparation of the living radical polymerization technique.
Another particularly advantageous feature is the fast response time. Response time is the time for building up of the diffraction grating in the photorefractive material when exposed to a laser writing beam. The response time of a sample of material may be measured by transient four-wave mixing (TFWM) experiments, as detailed in the Examples section below. The data may then be fitted with the following bi-exponential function:
xcex7(t)=xcex70(1xe2x88x92a1exe2x88x92t/J1xe2x88x92a2exe2x88x92t/J2)2
with a1+a2=1
where xcex7(t) is the diffraction efficiency at time t, xcex70 is the steady-state diffraction efficiency, and J1 and J2 are the grating build-up times. Between J1 and J2, the smaller number is defined as the response time.
Response time is important because the faster response time means faster grating build-up, which enables the photorefractive composition to be used for wider applications, such as real-time hologram applications.
Typical response times for known photorefractive materials range from seconds to sub-seconds. Times longer than 100 ms are common. Faster response times have been reported, see W. F. Moemer, Appl. Phys. Lett., Vol. 73, p. 1490 (1998), but, in order to get these higher speeds, higher field strengths have been required. Such higher field strengths may be difficult in an industrial, rather than a laboratory, environment. Also, the polyvinyl carbazole polymers used to obtain higher speeds become sticky and difficult to handle during heat processing. In contrast, the methacrylate-based, or more specifically acrylate-based polymers, that are preferred herein provide excellent workability during heat processing and other polymer handling methods.
In comparison with typical prior art materials, the photorefractive compositions of the invention provide good response times, such as no more than about 50 ms, and preferably faster, such as no more than about 40 ms, no more than about 35 ms, or no more than about 30 ms.
Furthermore, these response times can be achieved without resorting to a very high electric field, expressed as biased voltage. By a very high biased voltage, inventors mean a field in excess of about 100 V/xcexcm. In inventors"" materials, fast response times can generally be achieved at biased voltages no higher than about 100 V/xcexcm, more preferably no higher than about 90 V/xcexcm.
And, as discussed with respect to photoconductivity, these good response times can be provided in conjunction with one or more of the other advantageous properties as they are characterized above, such as high photoconductivity, low polydispersity, high diffraction efficiency, good processing capabilities, block copolymer capability, and efficient polymerization techniques.
Yet another advantageous feature is the diffraction efficiency. Diffraction efficiency is defined as the ratio of the intensity of the diffracted beam to the intensity of the incident probe beam, and is determined by measuring the intensities of the respective beams. Obviously, the closer to 100% is the ratio, the more efficient is the device.
In general, for a given photorefractive composition, a higher diffraction efficiency can be achieved by increasing the applied bias voltage.
In comparison with typical prior art materials, the photorefractive compositions of the invention provide good diffraction efficiencies. And, as discussed with respect to photoconductivity, these good diffraction efficiencies can be provided in conjunction with one or more of the other advantageous properties as they are characterized above, such as high photoconductivity, low polydispersity, or fast response time, and in conjunction with good processing capabilities, block copolymer capability, and efficient polymerization techniques.